alpha-vms: ETIR checks

alpha-vms: ETIR checks

Better validity checks, and remove a fuzzer vulnerability of sorts that
targeted the store-immediate-repeat command with a zero length but
very large repeat counts to chew cpu.

	* vms-alpha.c (_bfd_vms_slurp_etir): Check bound for the current
	command against cmd_length, not the end of record.  For
	ETIR__C_STO_IMMR check size against cmd_length, mask repeat count
	to 32-bits and break out on zero size.  Add ETIR__C_STC_LP_PSB
	cmd_length test.